We need a new roof. A builder suggested overlaying over the existing layer of shingles. Is it wise to do this? My concern is how would we know about any underlying damage? I would appreciate any input about the pros and cons of this method.
In general, it is not good practice to overlay a traditional asphalt shingle roof versus a complete tear off and replacement for the very reason you stated: you can’t examine the decking to determine if there is any damage that needs repaired. If you don’t take your roof down to the sheathing, you will never know if you have proper underlayment, or are experiencing sheathing rot, or ice or water damage.
That said, overlaying asphalt shingles with a metal roof could be an option, provided there is no underlying damage. Metal roofs offer energy savings but do cost more upfront than traditional asphalt shingles. They also last longer and stand up to foul weather better than shingles. No matter what type of roof you choose, make sure you tell your roofer to fix any underlying problems you have before installing any new roof.